Surface resistance of laser-deposited YBa2Cu3O7 films

Abstract

We have measured the millimeter-wave (100-GHz) surface resistance of high-quality laser-deposited ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ films on ${\mathrm{SrTiO}}_3$ and ${\mathrm{LaAlO}}_3$ substrates. Due to finite film thickness, radiation losses are important in the normal state and in the superconducting state near ${\mathit{T}}_{\mathit{c}}$. These effects are calculated, and ${\mathit{R}}_{\mathit{s}}$ characteristics of the ohmic losses in the film are extracted from the data. ${\mathit{R}}_{\mathit{s}}$ drops rapidly at ${\mathit{T}}_{\mathit{c}}$, and a detailed comparison with calculations that include finite mean-free-path effects suggests a gap that opens more rapidly as T/${\mathit{T}}_{\mathit{c}}$ decreases and attains a larger 2Δ(0)/${\mathit{k}}_{\mathit{B}}$ ${\mathit{T}}_{\mathit{c}}$ ratio than the standard BCS behavior.